In a base-station installation, it is desirable to be able to predict failures before the failures occur since this allows replacements and/or repairs to be carried out at scheduled maintenance visits. In the case of an antenna-embedded radio station, replacements and/or repairs may be conveniently carried out at times when the site is down (for example because another sharer of the site needs to undertake an antenna swap). The ability to control when maintenance is performed results in better reliability and lower cost of ownership.
In a base transceiver station or base-station, a transmission path typically comprises a modulator, a filter, an amplifier, and an antenna. Depending on the intended purpose of the transmission path, the amplifier needs to have a certain power rating so that the signal transmitted from the transmit path attains a desired range. In general the last amplifier in a transmission chain is termed a “power amplifier”. The last amplifier typically requires most attention to power efficiency. Efficiency considerations may cause the transmission path designer to choose a power amplifier that has certain drawbacks in some of the other properties of the power amplifier. Most of the time, a compromise between power efficiency and linearity of the power amplifier needs to be made. Thus, the power amplifiers may not be optimally configured with respect to linearity.
If the type and the characteristics of the non-linearity of the power amplifier are known, it is possible to process an input signal to the power amplifier in a manner compensating for the non-linear behaviour of the power amplifier. This technique is known as “predistortion”. A predistortion unit effectively models an inverse of the power amplifier's behaviour in order to achieve the desired compensation.
The predistortion can be achieved in a number of ways, including using analogue components or circuits. Digital predistortion (DPD) is nowadays often used if the signal to be amplified is available in a digital format.
The components of the transmission path are often subject to aging conditions and deterioration, as well as fluctuating environmental conditions, such as temperature and humidity. Digital components may be able to cope with effects of ageing in a satisfactory manner because of sufficiently large operating range margins facilitating reliable operation of the digital components. This is not always true for analogue components. The power amplifier in the transmission path exhibits an analogue behaviour, at least in part. Therefore, the operation of the power amplifier may change due to the aging and/or the fluctuating environmental conditions. If the power amplifier is preceded by the predistortion unit, the predistortion unit needs to reflect the changes in the power amplifier's behaviour in order to correctly compensate for these changes.
Another effect of the aging is that the components in the transmission path may begin to malfunction or fail after a certain period of time. The power amplifier is prone to suffer the ageing because it is often operated close to the limits of its specification, further adding to the stress to which the power amplifier is exposed.
The Japanese patent application JP2003-057273-A discloses a method for predicting an operation failure of a power amplifier. A distorted signal is extracted and converted into an IMD (intermodulation distortion) value to indicate the degree of distortion of a RF (radio frequency) signal from the power amplifier. The IMD value is monitored to check the degree of operation failure conditions of the power amplifier. The entire disclosure of JP2003-057273-A is incorporated herein by reference.